I... don't know. I don't think there is an answer. Wow. That is weird.
I mean "People will definitely..." means you're SURE it will happen. It's really common.
"People won't definitely..." means you're NOT sure it will happen, but you really need to put a loooot of spoken emphasis on the "definitely" part for it to make sense. Also, this way of saying it isn't so common, but people do say it. I don't think it's ever written though, because it's not good grammar. I guess we'd rather write "It's not definite whether people will work longer hours in the future."
When it's "People definitely won't..." it means you're SURE that it will NOT happen. Which is different from not being sure about it happening.
Lastly, I do sometimes hear "People definitely will...", but you're implying that YOU are SURE while OTHERS are UNSURE (spoken emphasis on the "will"). And the sentence usually changes, too ("People definitely will be working longer hours in the future.") It doesn't HAVE to change, but it's better grammar if it does. You're usually implying that the fact that this is happening is something you've planned, though.
Person 1: Will *I* get any icecream?
Person 2: You definitely WILL!
Does that make sense?